The term "wideband signals" is defined to mean digital signals having a range of 1.544 million bits per second (Mbps) to 6 Mbps or full analog baseband color video signals. The high frequencies required for wideband transmission, as compared to those required for voice transmission, result in more severe crosstalk problems within the switching network. In particular, when a space division network is using a crosspoint array which is fabricated using semiconductor technology, the crosstalk problems become increasingly more pronounced at higher frequencies. There are two main contributors to crosstalk problems in a semiconductor crosspoint array. One is the leakage from the conductors within the semiconductor array to the substrate caused by the relatively poor insulating qualities of the substrate and the capacitive effect due to the close proximity of signals within the semiconductor array. The second factor is that a crosspoint's switching transistor's internal structure has capacitive coupling between the input and output when in the OFF state. At higher frequencies, this capacitance becomes a lower and lower impedance which results in increased crosstalk. As a signal is switched through a normal multistage telephone-type network, these factors cause the crosstalk to increase in a cumulative, linear manner. The total crosstalk is equal to the sum of the crosstalk for each stage of the network within the network.
The approach taken by the prior art to eliminate this crosstalk problem has been to concentrate on the crosspoints themselves within the semiconductor array and attempt to reduce the OFF state capacitance of the crosspoint, to increase the dielectric isolation to the substrate and to physically distribute the conductors within the semiconductor array in a manner so as to reduce the capacitive coupling between conductors. These techniques have not been successful due to the basic limitations of the semiconductor devices and the fact that using these techniques has led to larger semiconductor chips, hence, lower yields and higher costs.
In view of the foregoing, there exists a need for a switching network architecture which can utilize semiconductor arrays which are easy to manufacture and meet the necessary crosstalk requirements.